Today, CDs (Compact Discs), DVDs (Digital Versatile Discs), BDs (Blu-ray Discs) and the like are produced on a commercial basis and are widely used as optical discs that are optical information recording media. The optical discs include various types such as a ROM (Read Only Memory) type which allows reproduction only, an R (Recordable) type which allows data addition, and an RE (Rewritable) type which allows information alteration. Information recording and reproduction in the optical disc is performed by an optical disc drive which is an optical information recorder irradiating the optical disc with a laser beam. Information recording is achieved by changing irradiation power of the laser beam and forming a mark having modified optical properties on a data layer of the optical disc. Information reproduction is achieved by irradiating the optical disc with a laser beam whose power is lower than that of mark formation on the data layer of the optical disc and detecting a difference in reflected light amount at every irradiation position.
The optimum write power used in information recording is different depending on optical disc types and manufacturers of the optical discs, and so the drive needs to set the optimum write power in accordance with the kinds of the optical discs. However, even the optical discs of the same kind are different from each other in the optimum write power due to production variations, and sometimes optimal recording may not be achieved if the same write power is used. Even in the case of using the same optical disc, sometimes optimal recording may not be achieved if the same set power is used due to power variations among drives. Accordingly, each drive is configured to perform trial writing in a specified area of the optical disc prior to optical disc recording and to adjust the optimal write power corresponding to the optical disc.
As a method for adjusting write power by using trial writing, there has been proposed, for example, a method for BD-RE using a relation between write power and modulation, which is disclosed in Non Patent Literature 1 that is a document of specifications for BD-RE.
In this method, an optimum write power Pw_opt is calculated with use of a relation between a write power Pw and a modulation M as well as predetermined parameters including a specified write power Pind, a coefficient target value κ and a coefficient ρ. These parameters are pre-recorded onto a control area of the optical disc. First, a specified signal is recorded onto a specified area of the optical disc with use of a plurality of types of write powers Pw[m] (m being an integer) in the vicinity of the specified write power Pind. By reproducing the recorded signal, a modulation M[m] which is a value obtained by dividing the amplitude of the reproduction signal by a higher envelope level is associated with the write power Pw[m] and acquired. Next, a relation between an evaluation value M[m]×Pw[m] and the write power Pw[m] is subjected to linear approximation in a specified range having arbitrary Pw[m] as a center, and a write power value at the point where the approximation straight line intersects a Pw axis (at the point where M×Pw becomes zero) is calculated as a write power threshold Pthr[m]. In the relation between a target write power Ptarget[m]=κ×Pthr[m] obtained by multiplying the calculated Pthr[m] by the coefficient target value κ and the write power Pw[m], a write power Pw which satisfies Ptarget=Pw is determined as an optimal target write power Ptarget_opt, and a value obtained by multiplying the optimal target write power Ptarget_opt by the coefficient ρ is determined as an optimum write power Pw_opt. The square brackets ([ ]) in the above description are used as a suffix for distinguishing each value when one index (e.g., Pw and M) has a plurality of values. The suffix may also be expressed with a character for collectively indicating these values (E.g., m). The index without the brackets (e.g., Pw in comparison to Pw[m] and M in comparison to M[m]) is a generalized expression of each index. This way of expression applies hereafter.
According to the method described above, each drive can set the optimum write power for each optical disc, and therefore optimal recording can be implemented. Since the write power adjustment method disclosed in Non Patent Literature 1 is a model performed with use of the coefficient target value κ, this write power adjustment method is hereinafter referred to as a κ model.
Another write power adjustment method using the relation between write power Pw and modulation M, which is different from the aforementioned method, is disclosed in Patent Literature 1.
In this method, an optimum write power Pw_opt is calculated by using a ratio α=Pw_opt/Pth between an optimum write power Pw_opt and a write power threshold Pth which are predetermined parameters. First, a specified signal is recorded onto a specified area of the optical disc with use of write powers Pw[m] (m being an integer) of a plurality of types. A modulation M[m] corresponding to each write power Pw[m] is acquired by reproducing the recorded signal. Next, write power correction values Pc[n] (n being an integer) of a plurality of kinds are set, and Pc[n] which makes a relation between an evaluation value M[m]×(Pw[m]−Pc[n]) and corrected write power (Pw[m]−Pc[n]) closest to a straight line is determined as an optimum write power correction value Pc_opt. A relation between an evaluation value M[m]×(Pw[m]−Pc_opt) and a corrected write power (Pw[m]−Pc_opt) is subjected to linear approximation, and the corrected write power value at the point where the approximation straight line intersects the axis (Pw−Pc_opt) (at the point where M×(Pw−Pc_opt) becomes zero) is calculated as a corrected write power threshold Pth′. A write power threshold Pth=Pth′+Pc_opt is calculated by using the calculated Pth′, and the calculated Pth is multiplied by a coefficient α to determine an optimum write power Pw_opt=α×Pth.
When this method is used, it becomes possible to determine optimal reproduction power with high precision and to thereby implement optimal recording for an optical disc having a curvilinear relation between M×Pw and Pw and for a medium having a modulation which uniformly changes depending on the state and reproduction condition of the optical disc (having a magnification which uniformly changes in all the measurement points).